The present invention relates to an odd-cylinder V-type internal combustion engine and, more specifically, to an arrangement for preventing a primary vibration caused by a primary inertial force acting on a reciprocating member, such as a piston.
A reciprocating internal combustion engine is provided with a balancing mechanism including a balance shaft to prevent vibrations due to unbalanced inertial forces acting on reciprocating members, such as pistons. The balancing mechanism including the balance shaft is driven through a transmission mechanism including a chain and gears by a crankshaft included in the internal combustion engine in synchronism with the rotation of the crankshaft. A balancing mechanism for an odd-cylinder V-type internal combustion engine is disclosed in JP-B 36252/1992. A V-type internal combustion engine mentioned in JP-B 36252/1992 is a five-cylinder internal combustion engine having three front cylinders disposed in a front part of a body frame, and two rear cylinders disposed in a rear part of the body frame. Connecting rods associated with the front cylinders, and those associated with the rear cylinders are connected to the two crankpins of the crankshaft, respectively, and a connecting rod associated with the leftmost cylinder, i.e., the cylinder at the left end of the V-type internal combustion engine as mounted on a motorcycle with respect to the forward traveling direction of the motorcycle, is connected to a crankpin at the left end of the crankshaft. A balancing mechanism is combined with the large end of the connecting rod associated with the leftmost front cylinder to counterbalance an inertial force generated when the internal combustion engine operates. The balancing mechanism includes two arms that perform a pendular motion when the large end turns. The balancing mechanism is held in a case disposed on the left side of the rear cylinders.
This known technique needs the balancing mechanism to counterbalance the inertial force and needs a space to hold the balancing mechanism. Consequently, the size and weight of the internal combustion engine are large, additional parts are necessary, and friction loss increases due to increase in sliding parts.
The present invention has been made in view of such circumstances and it is a first object of the present invention to provide an odd-cylinder V-type internal combustion engine capable of preventing the generation of a primary vibration due to unbalanced primary inertial forces without entailing increase in the weight and size thereof.
A second object of the present invention is to prevent the generation of a primary vibration by using a piston fitted in one of odd cylinders, not paired with any cylinder and to equalize primary inertial forces acting in the rotating direction of a crankshaft for the cylinders.
A third object of the present invention is to provide a simple arrangement capable of preventing a coupling vibration that is generated by primary inertial forces acting as a couple.
To achieve the first object, according to the present invention, an odd-cylinder V-type internal combustion engine includes 2n+1 cylinders (n is a natural number) arranged in two banks set in a V-shape, pistons fitted in the cylinder bores of the cylinders, connecting rods connected to the pistons, and a crankshaft provided with n common crankpins each connected to the connecting rods connected to two paired ones of the pistons respectively belonging to the two banks, and one odd crankpin connected to the connecting rod connected to the one unpaired piston; wherein the n common crankpins are disposed at the same phase, an angle xcex8 between the two banks (hereinafter referred to as xe2x80x9cbank angle xcex8xe2x80x9d) meets: xcex8=cosxe2x88x921(xc2xdn), the odd crankpin is disposed (180xe2x88x92xcex8)xc2x0 behind the common crank pins with respect to the rotating direction of the crankshaft when the unpaired piston belongs to the front bank with respect to the rotating direction of the crankshaft or (180xe2x88x92xcex8)xc2x0 ahead of the common crank pins with respect to the rotating direction of the crankshaft when the unpaired piston belongs to the rear bank with respect to the rotating direction of the crankshaft, reciprocating parts including all the pistons fitted in the cylinders, respectively, have the same mass, and the crankshaft is provided with balance weight which generates a balancing force of kMrxcfx892 acting in a sense of (xcex1+180)xc2x0, where:       k    =                                        (                                          1                2                            ⁢                                                1                  -                                      1                                          4                      ⁢                                              n                        2                                                                                                                  )                    2                +                              (                          n              -                              1                                  4                  ⁢                  n                                                      )                    2                          α    =                  sin                  -          1                    (                                    1            2                    ⁢                                    1              -                              1                                  4                  ⁢                                      n                    2                                                                                      k            )      
xcex1: Angle from the common crankpins in the rotating direction
M: Mass of each reciprocating part
r: Crank throw and
xcfx89: Angular velocity of the crankshaft.
In the odd-cylinder V-type internal combustion engine according to the present invention, in which the bank of the n+1 cylinders and that of n cylinders form the bank angle xcex8, primary inertial forces acting on the reciprocating parts including the 2n+1 pistons, while the internal combustion engine is in operation are counterbalanced with a balancing force of kMrxcfx892 acting in the sense of (xcex1+180)xc2x0 and, consequently, the generation of a primary vibration attributable to the primary inertial forces can be prevented. Thus, the generation of the primary vibration is prevented by the balance weight incorporated into the crankshaft, so that the balancing mechanism including the arms of the prior art or the balancing mechanism including the transmission mechanism and the balancing shaft is unnecessary, increase in the weight of the internal combustion engine can be suppressed, and the internal combustion engine can be formed in a small size. Reduction in the number of parts improves productivity, reduction in moving parts reduces friction loss because of mechanical friction between the moving parts and, consequently, the effective output of the internal combustion engine can be increased.
According to the present invention, the balance weight may include a plurality of balance weights, attributes of the balance weights may be determined so that the balance ratio for the reciprocating parts including each piston is 50%, and may be disposed diametrically opposite to the corresponding crankpin with respect to the axis of rotation of the crankshaft.
Thus, the unbalanced primary inertial forces acting on all the reciprocating parts including the pistons, connected to the common crankpins are counterbalanced with an unbalanced primary inertial force acting on the reciprocating part including the piston, connected to the odd crankpin to prevent the generation of the primary vibration. The balance weight can be easily incorporated into the internal combustion engine by using the reciprocating part including the piston other than the reciprocating parts including the paired pistons connected to the common crankpins to prevent the generation of the primary vibration. Since the balance weight is determined such that the balance ratio for each reciprocating part including the piston, is 50%, the primary inertial forces of a sense in the rotating direction of the crankshaft can be equalized, the design of bearings supporting the crankshaft is facilitated, increase in weight for rigidity enhancement can be avoided and the internal combustion engine can be formed in lightweight construction.
In the odd-cylinder V-type internal combustion engine according to the present invention, n represents an even number, the balance weight may include a plurality of balance weights, the odd crankpin may be disposed at a position corresponding to a middle part of the crankshaft with respect to directions parallel to the axis of rotation of the crankshaft, the reciprocating parts including all the pistons, and all the balance weights may be arranged symmetrically with respect to an imaginary plane including the center axis of the cylinder bore in which the unpaired piston is fitted and perpendicular to the axis of rotation of the crankshaft.
Thus, the primary inertial forces acting on the crankshaft are counterbalanced with the balancing force generated by the balance weights, the primary inertial force acting on each reciprocating part including, and the balancing force acting on the balance weight are symmetrical with respect to the imaginary plane. Therefore, the primary inertial force and the balancing force do not form a couple, the generation of a coupling vibration because of the primary inertial forces and the balancing force can be prevented by the simple arrangement in which the odd crankpin, the reciprocating parts including the pistons, and the balance weights are disposed symmetrically with respect to the imaginary plane.